NUTRIENT FLUXES ON AN INTERTIDAL MUDFLAT IN MARENNES-OLERON BAY, AND INFLUENCE OF THE EMERSION PERIOD

Fluxes of nutrients (NH4+, NO3-, PO43- and Si(OH)(4)) were studied on an intertidal mudflat in Marennes-Oleron Bay, France, at two different seasons and at different times of the emersion period. Fluxes through the sediment-water interface were both calculated from vertical profi les of nutrient concentration in porewater (diffusive fluxes, J(D)) an d measured in light and dark benthic mini-chambers (measured fluxes, J (0)). Results indicate that ammonia was mainly released in summer whil e nitrate was mainly taken up in late winter. This uptake from the ove rlying water was probably due to the coupling of nitrification-denitri fication within the sediment. The J(0)/J(D) ratio further indicates th at bioturbation likely enhanced ammonia release in summer. Concerning phosphate, the comparison of diffusive and measured fluxes suggests th at PO43- could be assimilated by the biofilm in winter while it was re leased in summer at a high rate due to both bioturbation and desorptio n because of the relative anoxic conditions in summer. Silica was alwa ys released by the sediment, but at a higher rate in summer. Statistic ally significant differences in measured fluxes were detected in dark chambers at different times of low tide, thus suggesting a short-term variability of fluxes. Microphytobenthos preferred ammonia to nitrate, but assimilated nitrate when ammonia was not available. It also turne d out that benthic cells could be Limited in nitrogen during low tide in late winter. In summer, ammonia was not Limiting and microphytobent hic activity significantly decreased the measured flux of NH4+ in the middle of low tide when its photosynthetic capacity was highest.